Hitherto, many golf balls have been commercially sold, but they are typically classified into thread wound golf balls and solid golf balls. The solid golf ball is further classified into a two-piece golf ball and a three-piece golf ball. The solid golf ball is generally approved or employed by most of amateur golfers, because it has better durability and better flight distance than the thread wound golf ball. On the other hand, the thread golf ball is generally approved or employed by professional golfers or high level amateur golfers, because it has a better shot feel and a higher spin amount than the solid golf ball.
The two-piece golf ball is generally inferior to the thread golf ball in shot feel and controllability on approach shots. The improvement in shot feel and controllability for approach shots in a two-piece golf ball has been intensely studied. It has also been proposed that its core is made of two layers, whereby the resulting golf ball has more excellent shot feel than the two-piece golf ball.
Golf balls having a two-layer structured core are described, for example, in Japanese Patent Kokai Publication Nos. 241464/1985, 181069/1987 and 80377/1989. These golf balls have one common structural feature, that is, that the hardness of the outer core is higher than that of the inner core. That is, making the inside of the core softer than the outside largely deforms the golf ball when hit by a club and provides the golf ball with a soft shot feel. However, a golf ball having such a structure has poor durability.
Japanese Patent Kokai Publication No. 23069/1994 proposes that the hardness of a core is made softer as it is farther from the boundary between the inner core and the outer core. In this structure, however, the inner core shows poor rebound characteristics and reduces the flight distance of the golf ball, because the inside of the inner core is softer than the outside.